1. Field of the Invention
The present invention relates to a volume holographic memory.
2. Description of the Related Art
Writing and reading data into and from a volume holographic memory are generally carried out in the following procedures. Digital data is converted to ON/OFF signals of spatial light, such as a dot pattern image with a contrast on a flat panel surface, a diffracted light of this image data or a signal light is caused to interfere with a coherent reference light and the resultant interference pattern is written and recorded in a holographic memory, e.g., a rectangular parallelopiped recording medium. The image of a dot pattern that is reproduced by irradiating the same light as the reference light on this holographic memory is received by an array of photodetectors, the output signal of the photodetector array is processed and converted back to digital data by an electronic circuit, then this digital data is read out.
A Fourier transform hologram is frequently used in data recording in a holographic memory.
In general, an image is considered as various spatial frequency components multiplexed in various directions. The distribution of the multiplexed spatial frequency components of an image can be acquired mathematically by computing two-dimensional Fourier transform in a similar manner to the way a time-variable electric signal or acoustic signal can be broken into various sinusoidal components.
Optically acquiring the angular distribution of the amplitude of a diffracted light of a uniform parallel light incident to an image using the Fraunhofer's diffraction phenomenon is equivalent to mathematical acquisition of two-dimensional Fourier transform of the amplitude transmittance of that image. A Fourier transform hologram is what is obtained by forming the image of a diffracted light from an image which is irradiated with a coherent parallel light, i.e., a signal light, by means of a Fourier transform lens located apart by its focal distance, transforming the image to a distribution on the focal plane or Fourier plane, causing the distribution resulting from the Fourier transform with a coherent reference light, and then recording the resulting pattern as interference fringes on a photosensitive material which is applied on a flat plate.
Because a wave surface recorded in a Fourier transform hologram corresponds to a Fourier-transformed image, the wave surface should undergo inverse Fourier transform for wavefront reconstruction as an image. Inverse Fourier transform is equivalent to convergence by a Fourier transform lens of a diffracted light reconstructed by irradiating the same reference light on a Fourier transform hologram on a flat plate, and the distribution of the amplitude transmittance of the original image is reconstructed on the Fourier plane.
A planar Fourier transform hologram has such an advantage that recording redundancy can be increased because a hologram can be stored in a limited space and information is dispersed in space after being subjected to Fourier transform.
In addition to such a planar Fourier transform hologram, there is a volume hologram thicker than the planar one. Generally, the volume hologram is deemed advantageous over the planar type in recording a vast amount of information due to its capability of making the diffraction efficiency greater. In this volume holographic memory, two-dimensional image data is recorded page by page in a dispersed manner in the three-dimensional space of this recording medium.
Since image data is recorded only at the portion in the holographic memory where the signal light and the reference light intersect each other, it is possible to implement spatial multiplexed recording by properly shaping the lateral cross-sectional shape of the reference light according to the shape of the recording medium. With the use of the reference light being shaped into an elliptical light beam of 1 mm in the vertical direction and 4 mm in the horizontal direction, for example, multiplexed recording in the units of 1 mm in the vertical direction is possible. In this case, recording is done by matching the position of the signal light with that of the reference light.
Recording media capable of recording a three-dimensional interference pattern inside as its spatial change in refractive index are recently receiving attention as a volume holographic memory. Such recording media include a photorefractive crystal such as lithium niobate (LN).
The photorefractive effect is a phenomenon such that charges generated by optical excitation move inside a crystal, forming the spatial electric field which is combined with the linear electrooptic effect or the Pockels effect, thus changing the refractive index of the crystal. In ferroelectric crystals having the photorefractive effect, the refractive index varies in response even to a tiny light input pattern normally of over 1000 lines per 1 mm and the effect occurs in real time at a response speed of the microsecond to second order depending on the material. Because of this property, studies on various applications of ferroelectric crystals as development-free real-time hologram media are under way.
Coherent lights which are used as the recording light and reference light are mainly a laser beam. As the laser beam has a Gaussian distribution of intensity in the widthwise direction, the light at the foot portion of the Gaussian distribution is irradiated outside the desired position even on the assumption that light of a predetermined intensity is to be irradiated on a predetermined position inside a photosensitive body. Further, at the time of angular multiplexing, as the reference light runs on the low angle side with respect to the normal axis of the irradiation plane, it has a greater irradiation width with respect to the photosensitive body. The greater irradiation width on a photosensitive body, whose size is limited in order to increase the recording density for more efficient recording, causes scattering of light incident to the edge portion of the photosensitive body, particularly, discontinuous faces at a portion where flat surfaces intersect one another as in a polygonal body or polyhedron. This light scattering may be recorded in a hologram as noise which adversely affects the quality of a reconstructed image.
FIG. 3 is an exemplary diagram showing irradiation of the recording light and the reference light when a photosensitive body 1 has a rectangular parallelopiped shape. When the reference light and recording light are irradiated, light scattering occurs at an angled portion of the rectangular parallelopiped photosensitive body 1 or a face-intersecting portion. The scattered light is superimposed on the recording light or reference light and is recorded as noise.